• Gather all the ingredients.
  
• Add 3 cups neutral oil to a heavy-bottomed pot (I use a 2.75-quart Staub Cast Iron Round Cocotte). You‘ll need at least 1 inch (2.5 cm) of oil. Bring it to 350°F (180°C) over medium heat. Meanwhile, sprinkle 6 fillets Atlantic horse mackerel on both sides with ¼ tsp Diamond Crystal kosher salt. Set it aside while you prepare the breading ingredients.
  

To Bread the Fillets

• Prepare three separate deep dishes with ¼ cup all-purpose flour (plain flour), 1 large egg (50 g each w/o shell), and 1 cup panko (Japanese breadcrumbs). Whisk the egg until beaten. Now, bread the fish. First, hold the fish by the tail and dredge the fillet in flour (do not bread the tail). Dust off the excess.
  
• Next, dip the fillet in the beaten egg and coat well.
  
• Finally, dredge the fillet in the panko, gently pressing the breadcrumbs onto the fish. Gently shake off any excess panko, then place it on a tray or plate. Continue breading the other fillets.
  

To Deep-Fry

• Meanwhile, check that the oil temperature is 350°F (180°C) with an instant-read thermometer. Or, stick a wooden chopstick in the oil; you‘ll know it‘s hot enough if you see tiny bubbles appear around the tip. You could also drop a piece of panko into the oil; if it sinks halfway and immediately pops back up to the surface, then the oil is ready. To cook, add two fillets to the oil and fry for 2 minutes, flipping halfway through. Tip: Fry in batches and do not crowd the pot. Your ingredients should take up no more than half of the oil surface area at any one time. If you add too much food at once, the oil temperature will drop and the fish will absorb too much oil. Maintain the correct oil temperature throughout cooking. For more helpful tips, see my post How to Deep-Fry Food.
  
• After 1 minute, flip the fillets and fry for another minute or until both sides are golden brown. If your oil gets too hot, lower the stove’s heat or/and add the next batch to cool down the oil.
  
• Remove the fillets from the oil, holding them vertically over the pot for a few seconds to drain the excess. Then, transfer to a wire rack or tray lined with a paper towel to drain. Continue to fry the remaining fillets. In between batches, collect the crumbs from the oil with a fine-mesh skimmer. If left, these crumbs will burn and make the oil darker, so make sure to keep the oil clean throughout deep-frying. When ready to serve, transfer the Aji Fry to individual plates.
  

To Serve

• In Japan, we usually serve deep-fried food with a side of finely shredded cabbage. I love to use a cabbage slicer because it saves time and energy! Aji Fry is delicious served with a side of Worcestershire sauce, Homemade Tonkatsu Sauce (or any kind of Japanese sosu (sauce) you‘d like), and/or Homemade Tartar Sauce. Enjoy!
  

To Store

• Store the Aji Fry in an airtight container for up to 3 days in the refrigerator. Reheat in the oven at 350ºF (180ºC). To freeze, place the Aji Fry in a single layer on a baking sheet until solid. Transfer to an airtight container or resealable freezer bag and freeze for up to 3 months. Reheat directly from frozen.